Gettering of metallic impurities in photovoltaic silicon

McHugo, Scott A.; Hieslmair, H.; Weber, E. R.

doi:10.1007/s003390050453

Cited by 96 publications

(54 citation statements)

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“…Interactions between metals and structural defects can either be advantageous 73,74 or detrimental 18,28,75 for solar cell device performance, depending on the spatial density, location (especially with respect to the pn-junction), and character of structural defects within the device. If one could, during crystal growth, reliably control the types of structural defects and their densities, one would subsequently reduce the variability in the sizes and densities of metal silicide nanoprecipitates.…”

Section: Structural Defectsmentioning

confidence: 99%

Chemical natures and distributions of metal impurities in multicrystalline silicon materials

Buonassisi

Istratov

Pickett

et al. 2006

Progress in Photovoltaics

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175

106

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We present a comprehensive summary of our observations of metal-rich particles in multicrystalline silicon (mc-Si) solar cell materials from multiple vendors, including directionally-solidified ingot-grown, sheet, and ribbon, as well as multicrystalline float zone materials contaminated during growth. In each material, the elemental nature, chemical states, and distributions of metal-rich particles are assessed by synchrotron-based analytical x-ray microprobe techniques. Certain universal physical principles appear to govern the behavior of metals in nearly all materials: (a) Two types of metal-rich particles can be observed (metal silicide nanoprecipitates and metal-rich inclusions up to tens of microns in size, frequently oxidized), (b) spatial distributions of individual elements strongly depend on their solubility and diffusivity, and (c) strong interactions exist between metals and certain types of structural defects. Differences in the distribution and elemental nature of metal contamination between different mc-Si materials can largely be explained by variations in crystal

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Section: Structural Defectsmentioning

confidence: 99%

Chemical natures and distributions of metal impurities in multicrystalline silicon materials

Buonassisi

Istratov

Pickett

et al. 2006

Progress in Photovoltaics

Self Cite

175

106

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“…See [8,[11][12][13][14][15] for further detail. The chemical state and distribution of iron impurities evolve during high-temperature solar cell processing steps [8,[16][17][18][19] because of the exponential dependence of iron point defect solubility and diffusivity on temperature. As the different states of iron exert varying impacts on minority carrier lifetime [20], accurate modeling of iron evolution is critical to determining its impact on the finished device.…”

Section: Introductionmentioning

confidence: 99%

Building intuition of iron evolution during solar cell processing through analysis of different process models

et al. 2015

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An important aspect of Process Simulators for photovoltaics is prediction of defect evolution during device fabrication. Over the last twenty years, these tools have accelerated process optimization, and several Process Simulators for iron, a ubiquitous and deleterious impurity in silicon, have been developed. The diversity of these tools can make it difficult to build intuition about the physics governing iron behavior during processing. Thus, in one unified software environment and using self-consistent terminology, we combine and describe three of these Simulators. We vary structural defect distribution and iron precipitation equations to create eight distinct Models, which we then use to simulate different stages of processing. We find that the structural defect distribution influences the final interstitial iron concentration ([Fe,]) more strongly than the iron precipitation equations. We identify two regimes of iron behavior: (1) diffusivity-limited, in which iron evolution is kinetic ally limited and bulk [Fe,] predictions can vary by an order of magnitude or more, and (2) solubility-limited, in which iron evolution is near thermodynamic equilibrium and the Models yield similar results. This rigorous analysis provides new intuition that can inform Process Simulation, material, and process development, and it enables scientists and engineers to choose an appropriate level of Model complexity based on wafer type and quality, processing conditions, and available computation time.

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“…15,16 Significant redistribution of iron and other metal impurities in silicon is possible during the high-temperature processing of silicon for solar cellsduring crystal growth, 9,17 by phosphorus diffusion, 18,19 by extended anneals at temperatures where defects remain mobile (i.e., low-temperature annealing), [20][21][22][23][24][25] and even during metallization firing. 26,27 In this contribution, we present a cohesive assessment of the redistribution of total iron concentrations-both precipitated and interstitial-using modern industrial processing steps and relate it to the final impact on device performance.…”

mentioning

confidence: 99%

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

Fenning

Hofstetter

Bertoni

et al. 2013

Journal of Applied Physics

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A phosphorus diffusion gettering model is used to examine the efficacy of a standard gettering process on interstitial and precipitated iron in multicrystalline silicon. The model predicts a large concentration of precipitated iron remaining after standard gettering for most as-grown iron distributions. Although changes in the precipitated iron distribution are predicted to be small, the simulated post-processing interstitial iron concentration is predicted to depend strongly on the as-grown distribution of precipitates, indicating that precipitates must be considered as internal sources of contamination during processing. To inform and validate the model, the iron distributions before and after a standard phosphorus diffusion step are studied in samples from the bottom, middle, and top of an intentionally Fe-contaminated laboratory ingot. A census of iron-silicide precipitates taken by synchrotron-based X-ray fluorescence microscopy confirms the presence of a high density of iron-silicide precipitates both before and after phosphorus diffusion. A comparable precipitated iron distribution was measured in a sister wafer after hydrogenation during a firing step. The similar distributions of precipitated iron seen after each step in the solar cell process confirm that the effect of standard gettering on precipitated iron is strongly limited as predicted by simulation. Good agreement between the experimental and simulated data supports the hypothesis that gettering kinetics is governed by not only the total iron concentration but also by the distribution of precipitated iron. Finally, future directions based on the modeling are suggested for the improvement of effective minority carrier lifetime in multicrystalline silicon solar cells.

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Gettering of metallic impurities in photovoltaic silicon

Cited by 96 publications

References 0 publications

Chemical natures and distributions of metal impurities in multicrystalline silicon materials

Chemical natures and distributions of metal impurities in multicrystalline silicon materials

Building intuition of iron evolution during solar cell processing through analysis of different process models

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

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